1. Field of the Invention
This invention relates to a veritable prosthetic system and device and a method for implanting the device and, more particularly, to a spinal fusion system and method for fusing spinal bones. This invention also relates to a surgical implant that is used, in its preferred embodiment, for the support of spinal vertebrae. The implant comprises a cage element, at least two independent screw-traversing plate elements, and at least two screw elements, which are placed between the bodies of vertebrae in a surgical procedure. The implant further comprises a mechanism by which each screw-traversing plate element can independently move axially with respect to the cage element and at least one other screw-traversing plate element.
2. Description of the Related Art
Many types of prosthetic devices have been proposed in the past. For example, U.S. Pat. No. 5,192,327 to Brantagan concerns a surgical prosthetic modular implant used singularly or stacked together to support and fuse together adjacent vertebrae or to totally or partially replace one or more vertebrae in a vertebral column. Other surgical implant devices and methods are shown in U.S. Pat. Nos. 5,192,327; 5,261,911; 5,713,899; 5,776,196; 6,136,002; 6,159,245; 6,224,602; 6,258,089; 6,261,586; 6,264,655; 6,306,136; 6,328,738; 6,592,586; U.S. Patent Publication Nos. 2006/0195100; 2007/0123885 and 2008/0021476. Some or all of these devices have improved the success rate and have simplified the surgical techniques in inter-body veritable fusion.
Among some of the problems associated with the prior art devices is that after the device is inserted into a patient during a surgical procedure, there was a possibility of retropulsion of the inter-body device and graft material into the spinal cord or other neurological element.
Another problem with the prior art devices is that grafting material, which was inserted into the devices during the surgical procedure, could not easily be inserted from an anterior direction.
Moreover, in some of the prior art devices, the cover, if any, was typically fastened directly to the device and to spinal bones, which prevented the cover from being capable of moving relative to the device. In addition, in devices that used a cover, the cover did not function to both retain the grafting material in the device and simultaneously fix the spinal bones relative to each other.
Another problem with prior art cage systems is that the screws or fasteners which secured the cover onto the cages sometimes had a tendency to unscrew themselves which is undesirable because the graft material may exit the cage or because the cage itself may move. Another problem is that the screws may withdraw, causing injury to local structures by the screws themselves.
The field of spinal implantation burgeons with devices and methods for the achievement of fixation between vertebrae. Commonly, intervertebral “cages” are utilized to contain bone graft material and provide physical support between adjacent vertebrae. Such devices are exemplified by U.S. Pat. No. 6,371,986 to Bagby, and U.S. Pat. No. 5,609,637 to Biedermann. More recently, devices have been developed which provide supplemental fixation, typically in the form of screws, integrated in some manner with the cage. Such supplemental fixation helps to stabilize the device and prevent loosening until a natural bony fusion takes place. Such devices are exemplified by U.S. Pat. No. 4,904,261 to Dove and U.S. Pat. No. 6,432,106 to Fraser.
In designing devices which integrate a cage component and a screw component, a challenge has been the biomechanical forces between these components. After placement of any cage device, a certain amount of settling or axial motion occurs between the cage and the adjacent vertebrae. Such motion is inevitable and even desirable as it places stabilizing compression onto the device. However, if the cage is rigidly affixed to the adjacent vertebrae with integral screws, such axial motion can be problematic. Undesirable outcomes include screw loosening or breakage as settling invariably occurs.
In order to avoid such outcomes, various mechanisms allow for multi-axial motion between the screw and cage components. One problem with this approach is that multi-axial motion will allow for screw rotation within the vertebral body, reducing the effectiveness of fixation. A further disadvantage is that the multi-axial approach prevents a fixed, repeatable screw trajectory to ensure appropriate placement into the adjacent vertebral body.
What is therefore needed is a device incorporating screw-cage interfaces which provide for controlled uni-axial motion which allows for axial settling, but will not produce undesirable rotation within the vertebral body. Furthermore, the system needs to provide a specific, repeatable screw trajectory to ensure appropriate placement into the adjacent vertebral body.
What is needed, therefore, is a system and method, which facilitates overcoming one or more of the aforementioned problems as well as other problems and to provide a device that has unique features that will facilitate reducing the risk associated with neurological surgeries and advance the present state of the art.